We are working on a new approach to treating acute arthritis based on a common pathway in the cellular infiltrates that damage bones and joints. Inflammatory arthritis disables thousands of people each year. It occurs in children and adults, after trauma and infections but most cases are idiopathic. Millions of adults live with chronic RA, which has similar features. Inflammatory arthritis often requires treatments with serious side effects. Later in arthritis, bone erosion is a major problem that causes severe pain and debilitation. There is no small molecule drug available to specifically treat arthritic bone erosion. We show that osteoclast maturation is suppressed by blocking a calcium-release activated calcium channel, called Orai. A Orai antagonist, N-methyl- 3,4-dichloropropionaniline (N-MeDCPA), suppressed osteoclast maturation and strongly suppressed collagen- induced arthritis (CIA) in mice, even after symptoms of arthritis were measureable. We propose that pharmacologically suppressing CRAC channels with N-MeDCPA will prevent bone erosion due to arthritic stimuli without major adverse effects. We hypothesize that Ca2+ signals mediated by Orai channels modulate final differentiation of osteoclasts, and that related effects on T cells might reinforce bone and joint sparing in acute arthritis. We expect toxicity of N-MeDCPA to be low when treatment is for several weeks in animals with mature immune systems. We propose comprehensive pharmacokinetic and toxicokinetic testing in addition to mechanistic experiments with Ca2+ signaling, immunology, and bone biology expertise. Phase I - AIM 1: - Drug formulation, route of administration optimization and Orai specificity. N- MeDCPA is lipid soluble. This aim will define a suitable nanoparticle (NP) or liposome formulation of N- MeDCPA for parenteral or oral administration. Drug blood levels versus ease/convenience and effectiveness will be assessed. Phase I ? AIM 2: - Complete the dose-response studies of N-MeDCPA using the CIA model. Having demonstrated that N-MeDCPA can prevent bone erosion and arrest inflammation in symptomatic mice, the emphasis of this aim will be to define its effect against the standard CIA measures, arthritic index, bone erosion, volume by CT and inflammation (swelling) using the optimized dosing regimen. Phase II - AIM 1: Pharmacology and toxicology assessment of N-MeDCPA under non-GLP and GLP conditions in vitro and in vivo. An extensive suite of tests will be performed by a contract research organization (CRO) under contract with ExesaLibero (see letter of support from Covance). Phase II - AIM 2: Determine the therapeutic-potential of N-MeDCPA. Critical to demonstrating that N- MeDCPA will improve the standard of care/quality of life, is confirmation of equal effectiveness in both sexes using SKG mice, determining its cellular specificity, checking for potential immunotoxicity of N-MeDCPA in tests not covered by the CRO and exploring possible additive/synergistic effects of N-MeDCPA in combination with other current non-biological treatments, e.g., methotrexate. We will also study the in vitro effects on human cells to avoid the pitfall of rodent-human physiological differences. Phase II - AIM 3: Assess the specificity and mechanism of action of N-MeDCPA. We will assess the action of the N-MeDCPA on monocyte differentiation to osteoclasts, and investigate the possible side effects on osteoblasts and its specificity for ion channels. The development of N-MeDCPA as a drug for treatment of bone erosion promises to prevent the life- changing debilitation associated with RA, effectively with low toxicity.